I'll always slow down and look, too. After all these years, can't help but do it - Malcolm phrased it well: a moral imperative to become involved, if necessary (which is a big reason a lot of us became LEOs).

As Evan, Ayoob, and others have written, plainclothes intervention/assistance is very risky, so the manner of approach is essential. In addition to always being armed, I always have at hand in my POV a ball cap with POLICE in large letters on the front, and a badge on a badge holder with neck chain.